1. Field of the Invention
The present invention relates to a method for forming an array of antifuses and, more particularly, to a method for forming an array of sidewall-contacted antifuses having diffused bit lines.
2. Description of the Related Art
Unlike a fuse which, when programmed, changes from a low-resistance to a high-resistance device to block a current from flowing through the device, an antifuse is a device which, when programmed, changes from a high-resistance to a low-resistance device to allow a current to flow through the device.
FIG. 1 shows a plan drawing that illustrates a portion of a conventional antifuse array 100. FIG. 2 shows a cross-sectional drawing taken along line 2--2 of FIG. 1. As shown in FIGS. 1 and 2, array 100 includes a series of polysilicon (poly) bit lines BL1-BLm which are formed on a first layer of oxide 110. As further shown, each bit line BL has a series of projections P1-Pn which, in turn, each have an end sidewall 112.
In addition, array 100 further includes a second layer of oxide 114 which is formed over bit lines BL1-BLm and oxide layer 110, but which is not formed on the end sidewalls 112 and the regions of oxide layer 110 that lie between opposing end sidewalls 112.
Array 100 also includes a series of word lines WL1-WLm which are formed on oxide layers 110 and 114 and the end sidewalls 112 such that a word line WL is connected to all of the end sidewalls 112 in a row of end sidewalls. Each word line WL, in turn, includes a dielectric layer DL and an overlying poly layer PL.
The resulting structure forms a plurality of sidewall-contacted antifuses 116 in that an antifuse 116 is formed each time a word line WL contacts the end sidewall 112 of a bit line BL.
In operation, each antifuse 116 is used to store one of two logic values depending on whether or not the dielectric associated with the antifuse is intact. For example, antifuse A represents a first logic state when the dielectric of word line WL1 formed over end sidewall 112 of bit line BL1 is intact in that no current can flow from word line WL1 to bit line BL1 (or in the opposite direction). Thus, when normal operating voltages are applied to word line WL1, only a fraction of the voltage is coupled to bit line BL1.
On the other hand, antifuse A represents a second logic state when the dielectric of word line WL1 formed over end sidewall 112 of bit line BL1 has been broken down and thus, is no longer intact, in that current can now flow from word line WL1 to bit line BL1 (or in the opposite direction). Thus, when normal operating voltages are applied to word line WL1, substantially all of the voltage is present on bit line BL1.
The dielectric region of a word line that forms an antifuse is typically broken down by establishing a strong electric field across the dielectric region. This programming of the antifuse changes the logic state of the antifuse from the first logic state to the second logic state.
For example, with antifuse A, the dielectric of word line WL1 formed over end sidewall 112 of bit line BL1 breaks down when a 10.6V pulse is applied to word line WL1 while bit line BL1 is grounded.
One of the problems with antifuse array 100 is that antifuses 116 are sensitive to masking alignment errors that occur during fabrication. FIGS. 3A-3F show a series of plan views that illustrate the fabrication of antifuse array 100. (See also Chen et al., A Sublithographic Antifuse Structure for Field Programmable Gate Array Applications, IEEE Electron Device Letters, Vol. 13, No. 1, January 1992.)
As shown in FIG. 3A, antifuse array 100 is fabricated by depositing a layer of first polysilicon (poly-1) 310 over a layer of oxide 312. After this, poly-1 layer 310 is masked and etched to form a poly structure 314 which includes a first strip 316, a second strip 318, and a plurality of connecting strips 320 that connect the first and second strips 316 and 318 together.
Following this, poly-1 layer 310 is again masked and etched to reduce the widths W of connecting strips 320 to a dimension which is less than the minimum dimension that can be photolithographically obtained by the fabrication process.
Following this, as shown in FIG. 3B, a thick layer of oxide 322 is formed over strips 316, 318, and 320, and the exposed portions of oxide layer 312. Next, a mask 324 is formed and patterned on oxide layer 322 to expose the regions of oxide layer 322 that are formed over the center portions of connecting strips 320. After this, the unmasked areas of oxide layer 322 and the underlying portions of strips 320 are etched to form bit lines BL1-BLm.
However, as shown in FIG. 3C, if mask 324 is vertically misaligned, then the etch will fail to form bit lines BL1-BLm. Instead, the etch will leave connecting strips 320 partially or fully intact, depending on the extent of the misalignment.
As shown in FIG. 3D, if mask 324 is properly aligned, the next step following the last etch is to form a layer of dielectric material 326 on the top and sidewall surfaces of oxide layer 322, the end sidewalls of bit lines BL1-BLm, and a top surface of oxide layer 312.
Next, a layer of second polysilicon (poly-2) 328 is formed over dielectric layer 326. Following this, a mask 330 is formed and patterned over poly-2 layer 328. The unmasked areas of poly-2 layer 328 and the underlying areas of dielectric layer 326 are then etched to form word lines WL1-WLn as shown in FIG. 3E.
However, as shown in FIG. 3F, if mask 330 is vertically misaligned, then the etch will fail to form word lines WL1-WLn over connecting strips 320. Thus, alignment errors during the placement of masks 324 and 330 can result in the formation of non-functioning antifuses.
As a result, there is a need for a method for forming an antifuse array which is less sensitive to masking alignment errors.